View full size(Press-Register/Ben Raines)Swirls of oil float at the edge of stained marsh grass inside Horn Island's Garden Pond, one of several marshes on the island's interior, on August 8, 2010. The oil penetrated deep into the green marsh grass, coating the stalks from the mud to about 18 inches up.

HORN ISLAND, Miss. -- Weeks after BP capped its runaway well, a greasy band of oil appeared on the grasses fringing Garden Pond, a previously pristine interior marsh.

Glops of deep brown oil floated on the surface of the saltwater pond Saturday and appeared to be scarcely weathered, compared to much of the oil that has come ashore. The oil penetrated deep into the green marsh grass, coating the stalks from the mud to about 18 inches up.

The stain on the grass around the pond looked like a dirty black ring that one might see in a bathtub, although the grass still seemed vigorous.

Horn Island is part of the Gulf Islands National Seashore run by the National Park Service, which includes Petit Bois, Ship and Cat islands, as well as land in Florida. The undeveloped island is open to the public but accessible only by boat.

Horn lies about 10 miles south of Pascagoula and is popular with birders because of the diverse habitat, ranging from forested areas to dune fields covered in prickly pear cactuses.

Made famous by the artwork of Walter Anderson, Horn Island's numerous salt ponds are connected to the Mississippi Sound by small creeks that allow the tide to flush in and out. Park officials said the tide likely carried the oil into the marsh sometime in the last week, with much of it coming in underwater instead of floating on the surface.

Rows of boom surrounding the mouth of the creek appeared not to have any oil on them. Such boom can catch only floating oil.

Mats of floating oil were present in the Garden Pond, as well as in the narrow channels that finger into the marsh.

Marsh periwinkles, the small snails found in coastal marshes by the thousands, could be seen clinging to the grass stems, bits of sticky oil present on their shells. Hermit crabs prowled the water's edge, their shells also stained.

Clumps of oysters in the creek were smeared with a goopy layer of oil. A large blue crab encountered by the Press-Register raised its claws in defense of a small pool of water when approached but didn't attempt to flee or bury itself.

Encountered again about 30 minutes later, the crab had expired, its claws hanging limp, small clumps of oil clinging to the mouth parts used for breathing and eating.

While there was a good deal of oil present in the pond, cleaning it would be challenging, as it coated every stalk of grass, every piece of driftwood and the mud bottom that the plants are growing from.

Ed Cake, a Mississippi biologist, visited the island over the weekend. He said that machinery is not typically allowed in wild areas like Horn Island and noted that few tools in the cleanup arsenal would be suited to the marsh, with the possible exception of various microbes and enzymes designed to degrade oil in delicate habitats.

Some of the large glops of oil floating in the creeks would best be collected by hand, he said.

"I think that marsh will recover on its own, but slowly. The oil is already turning to sheen and flowing out of there. The animals are stressed but mostly surviving," Cake said. "It looks bad, but I'm not totally disheartened. It will be a great opportunity to study how these places recover. You don't want anybody trying to get in there and clean that stuff up with machines. They will do more damage than the oil."

The Press-Register explored a few of the other marsh ponds along the island but did not find oil in them. Garden Pond is fairly close to the east end of the island, which received heavy doses of oil several times during the last several months.

Crews have cleaned the Horn beaches, but tarballs are still easy to find around the island's tip, both buried in the sand and rolling in the surf, some as large as softballs.

The grass beds just offshore of the island appeared healthy and oil free, inhabited by crabs, shrimp and small fish.